1. Field of the Invention
The present invention relates to a peripherally driven veneer lathe and an improvement of a knife used therefor.
2. Prior Art
Recent years have witnessed so-called peripherally driven veneer lathes becoming the mainstream in the field of veneer lathes. The peripherally driven veneer lathe is designed such that at least part of the power required for peeling a log is supplied through the periphery of the log. Examples of this type of lathe are disclosed in, e.g., Japanese Patent Examined Publication (Kokoku) No. 59-28444, Japanese Registered Utility Model No. 2539258, Japanese Patent Examined Publication (Kokoku) No. 61-21808, and U.S. Pat. No. 6,357,496. The veneer lathe of this type comprises a peripheral-drive member having a plurality of drive members disposed at proper intervals along the axis, each drive member having on its periphery a number of piercing projections. The peripheral-drive member is disposed such that the piercing projections can pierce the periphery of the log immediately before the cutting edge of the knife. Thus, this type of lathe can transmit motive power to the log via the piercing projections in an extremely stable manner, resulting in its very wide use.
The peripherally driven veneer lathe of the above-described type will now be described by referring to an actual example shown in the drawings. As shown in FIG. 8 and FIG. 9, the latter being an enlarged view of a portion indicated by a circle B in FIG. 8, the peripherally driven veneer lathe comprises a knife 2, a knife carriage 1, a peripheral-drive member 4, and a spindle 3 for supporting a log 5. The knife 2 comprises a cutting face 2a and a flank 2b and is mounted on the knife carriage such that the flank is opposite the log. The peripheral-drive member 4 comprises a plurality of drive members 4b disposed at proper intervals along the axis, each drive member 4b being disposed substantially parallel to a cutting edge 2c of the knife 2 and having a number of piercing projections 4a on the periphery. Normally, the peripheral-drive member 4 is disposed at such a position that the piercing projections 4a can pierce the periphery of the log immediately before the cutting edge 2c of the knife 2, which is mounted on the knife carriage 1 via a knife clamp 1a. A veneer 6 is produced by providing at least part of the motive power necessary for peeling the log 5, held by a spindle 3, from the peripheral-drive member 4. This manner of supplying the power makes it possible to peel both an unusually hard and an unusually soft log with ease, which was very difficult for the conventional spindle-driven veneer lathes.
An improved example in practical use will be described by referring to FIG. 11 and FIG. 12, the latter showing an enlarged view of a portion of FIG. 11 indicated by a circle C. In this example, the function of the piercing projections 4a is secondarily exploited. As shown, in addition to the features of the above-described example, this example includes a guide member 8 having a guide surface 8a mounted at the tip of the knife carriage 1. The guide surface 8a substantially coincides with a part of a circle (to be hereafter referred to as a concentric arc) concentric with a rotation trajectory G of the piercing projections 4a but with a larger radius and therefore away from the rotation trajectory G by a predetermined distance. This example further includes a bending member 9 disposed between adjacent drive members 4b, by which the veneer 6 is forcibly bent toward the back side, thereby creating many surface cracks starting from the points pierced by the piercing projections 4a and providing the veneer 6 with flexibility. Another improved example (not shown) in practical use includes, in addition to the features of the above example, backup rolls for holding the periphery of the log from at least two directions, so that the log can be released from the spindle in the final stage of peeling and peeled until it becomes thinner than the spindle.
In this type of veneer lathe, the peripheral-drive member 4 is disposed such that the piercing projections 4a can pierce the periphery of the log immediately before the cutting edge 2a of the knife 2 mainly because at this position, a preferable engagement can be obtained between the piercing projections 4 a of the peripheral-drive member 4 and the log 5. Specifically, in order to prevent the piercing projections 4a and the knife 2 from colliding with and damaging each other due to vibrations and the like of the peripheral-drive member 4, the peripheral-drive member 4 must be disposed in such a manner as to ensure a gap S (see FIGS. 9 and 12) of more than a certain limit (generally, about 1 mm) between the rotational trajectory G of the piercing projections 4a and the knife 2 when the knife 2 and the peripheral-drive member 4 are most closely located (the peripheral-drive member may be fixedly secured to the knife carriage 1, or it may be movably mounted thereon, as disclosed in the above-mentioned Japanese Registered Utility Model No. 2539258 or Japanese Patent Examined Publication (Kokoku) No. 61-21808). Also, the depth of piercing on the log 5 by the piercing projections 4a must be correspondingly controlled. Under these restrictions, the above position is preferable if the piercing projections 4a and the log 5 are to be satisfactorily engaged with each other as the log 5 is peeled and becomes smaller in diameter, as shown by the broken line in FIGS. 8 and 9. Another reason why the above position is selected is that positioning the peripheral-drive member 4 there enables the piercing projections 4a to pierce the veneer 6 as well, which facilitates the smooth delivery of the veneer 6.
In other words, in order to obtain a satisfactory engagement between the piercing projections of the peripheral-drive member and the log, it is effective to set the position of the peripheral-drive member 4 such that an axis 4d of an axle 4c of the peripheral-drive member 4 is located on or near a line perpendicular to the cutting face 2a and passing at the cutting edge 2c of the knife 2 (the line passing at the edge of the knife and normal to the cutting face), as shown in FIGS. 8 and 11. While not shown, when the peripheral-drive member is movably mounted, too, as disclosed in the above-mentioned Japanese Registered Utility Model No. 2539258 or Japanese Patent Examined Publication (Kokoku) No. 61-21808, the peripheral-drive member is preferably positioned at the position as described above when the knife and the peripheral-drive member are most closely located with each other.
If the peripheral-drive member is disposed at a higher position than the above position, the piercing projections can be located further away from the knife and thus a collision can be reliably avoided and the piercing projections can more deeply pierce the log. However, in this case, the position at which the motive power is supplied will be further away from the cutting edge of the knife, and this creates a problem in the driving of the log. For example, if a part of the log is missing or the periphery of a non-cylindrical log is to be peeled, or if the log is to be peeled down to an extremely small diameter, it becomes difficult to perform peeling at the end portion of the veneer, i.e., the end portion may remain uncut and get stuck on the knife, thereby causing the veneer to be torn at an arbitrary, undesirable location, such as one on the extension of a crack (formed in the log) that already exists on the veneer.
In the above-described peripherally driven veneer lathe, the shape of the knife and the manner in which it is mounted on the knife carriage are the same as in the conventional spindle-driven veneer lathe. In the conventional example, as shown in FIG. 11, the knife 2 includes two parallel surfaces 2a and 2d, and a surface 2b which connects the two surfaces 2a and 2d in an inclined manner and which, together with the surface 2a, forms the cutting edge 2c. From the viewpoint of ease of replacement of the knife, reliability (stability) with which the knife is held, and so on, the knife 2 is mounted such that the surface 2b is opposite the log 5 and forms a flank, while the surface 2a forms a cutting face, as shown. Further, as shown in FIG. 12, a sharpness angle xcex2 formed by the flank 2b and the cutting face 2a is set at a desired value (normally, in the range of from 18xc2x0 to 25xc2x0). If necessary, in order to improve, e.g., the wear resistance of the cutting edge, a microscopic portion of the cutting edge 2c (mainly a portion on the flank side extending from about 200 xcexcm to about 700 xcexcm) is finish-ground with one or more angles which are somewhat larger than the sharpness angle xcex2, before the knife is mounted on the knife carriage 1 with a required angle of relief xcex3 (normally, from 30xe2x80x2 to 1xc2x0) relative to a vertical line Y extending from the cutting edge 2c. 
The sharpness angle xcex2 is closely related to the performance of a knife. A decrease in the sharpness angle xcex2 increases the cutting ability of the knife but undermines its deflection resistance. Conversely, an increase in the sharpness angle enhances the deflection resistance but weakens the cutting ability. At any rate, since the deflection resistance of a knife with the above shape is restricted to be below a limit corresponding to the sharpness angle xcex2, the deflection resistance of the knife in the conventional machines has not always been good enough, resulting, for example, in the cutting edge portion (including the cutting edge 2c and a portion near it) being deflected in the course of peeling the log, as indicated by the dotted line in FIGS. 10 and 13 (the deflection occurs mainly toward the cutting face; but it may be toward the flank). As a result, the thickness of the produced veneers may vary, or the peeled surface of the veneer becomes coarse, for example. Moreover, once the cutting edge portion is deflected, a hard log tends to gradually increase the deflection, making, in a worst case, it impossible to continue the peeling operation.
It should be noted that even if the guide member 8 is provided as shown in FIGS. 11 and 12, if the edge of the guide member 8 is extended to the vicinity of the cutting edge 2c of the knife 2, the edge portion has to be made extremely thin. This makes it liable that, if the cutting edge portion of the knife is deflected away from the log even once, distortion remains in the edge portion of the guide member, which is deflected along with the cutting edge portion. Should even a hint of distortion remain there, the delivery of subsequent veneers will be hindered. Therefore, the edge of the guide member 8 has to be provided with a step and positioned significantly away from the cutting edge of the knife after all, as shown in the drawings. This means that the guide member 8 can hardly be expected to provide the function of preventing the deflection of the cutting edge portion of the knife. Furthermore, if the guide member is provided with a step, the direction of delivery of the veneer changes suddenly near the tip of the guide member, thereby hindering a smooth delivery of the veneer.
The present invention has been made with a view to overcoming the defects of the conventional peripherally driven veneer lathe. It is therefore an object of the present invention to enhance the deflection resistance of the cutting edge portion and improve its cutting ability by improving the shape of the cutting edge portion of the knife mounted on the knife carriage, while maintaining the positional relationships among the constituent members or elements as known in the art, so as not to adversely affect the log-driving function, the cutting ability and so on.
In accordance with the present invention, in the peripherally driven veneer lathe of the known type as described above, the cutting edge of the knife is positioned on the flank side a desired distance away from a point of intersection of an extended plane of the cutting face and an extended plane of the flank, toward the downstream of the rotation of the log, wherein the cutting edge and the cutting face are connected by a connecting curved surface. Further, the knife is mounted on the knife carriage such that the connecting curved surface is positioned away from the trajectory of rotation of piercing projections formed on a peripheral-drive member with a predetermined gap when the peripheral drive member is most closely positioned to the knife.
Preferably, the connecting curved surface of the knife mounted on the knife carriage is formed such that the angle formed by a line tangent to the connecting curved surface at the cutting edge and the flank is roughly the same as the angle of intersection of the extended plane of the cutting face and the extended plane of the flank. Preferably, the angle formed by the line tangent to the connecting curved surface at the cutting edge and the flank is in the range of from 18xc2x0 to 25xc2x0.
The curved surface of the knife may comprise a second curved surface closer to the cutting edge and a first curved surface that follows the second curved surface, the second curved surface having a greater curvature than the first curved surface, wherein the knife is mounted on the knife carriage such that the first curved surface is positioned away from the rotational trajectory of the piercing projections on the peripheral-drive member with a predetermined gap when the peripheral-drive member is most closely positioned to the knife.
In this case, it is preferable that the angle formed by a line tangent to the second curved surface at the cutting edge and the flank is in the range of from 8xc2x0 to 25xc2x0, and that the length of the second curved surface is in the range of from 1 to 5 mm. Further, a microscopic portion of the cutting edge of the knife mounted on the knife carriage may be provided with a finish grinding with one or more finish angles which are somewhat larger than the angle at an extreme edge portion of the cutting edge portion.
In a further preferable embodiment of the peripherally driven veneer lathe according to the present invention, in order to ensure a satisfactory cutting ability, the axis of rotation of the peripheral-drive member with the many piercing projections formed on its periphery is positioned on a line perpendicular to the cutting face and passing at the cutting edge of the knife, when the knife and the peripheral-drive member are most closely positioned to each other.